In International Patent Application No. PCT/US2014/018175 (WO 2014/130987), the disclosure of which is incorporated by reference in its entirety herein, processes are described for the oxidation of a hydrocarbon such as an alkane with an oxidizing electrophile comprising a main group element in an acidic medium. Processes for use of an oxidizing electrophile in an acidic medium for oxidation of heteroalkanes and arenes are described in U.S. Provisional Patent Application Nos. 62/041,270 and 62/042,101, also incorporated by reference in their entireties herein. In the methods of the inventions, a compound (RH), such as an alkane, heteroalkane, or arene, is contacted with an oxidizing electrophile comprising a reactive main group element (e.g., a compound of formula MXn, in which M is the main group element such as thallium, lead, bismuth, antimony, selenium, tellurium, or iodine, in an oxidized form or state, and X is a negatively charged counterion). The reactive main group element in oxidized form, as a salt, is termed an oxidizing electrophile (e.g., a soft oxidizing electrophile).
Without wishing to be bound by any particular theory, it is believed that this reaction generates a reactive organometallic intermediate (RMX(n-1)), which in at least some embodiments is derived from electrophilic substitution of a carbon-bonded hydrogen atom of an sp3- or sp2-hybridized carbon atom by the reactive main group element. The reactive organometallic intermediate, wherein a newly formed bond exists between the hydrocarbyl group and the main group element, can then further react, such as with an oxygen acid like a carboxylic acid, to yield a functionalized hydrocarbon, e.g., a hydrocarbyl ester of the carboxylic acid. Thus, the overall reaction is replacement of a hydrogen atom of a C—H bond of the compound with a functional group, such as an ester group. The byproduct of the oxidizing electrophile, termed an electrophile reduction product, comprises the main group element M in a lower oxidation state relative to the oxidation state of the main group element in the reactive oxidizing electrophile.
In an embodiment, the reaction is carried out in an acidic medium, which yields the reaction product and an electrophile reduction product from the reactive organometallic intermediate, wherein the main group element M has undergone a reduction reaction to a lower oxidation state and the substrate has been transformed to a functionalized (e.g., an oxidized) hydrocarbon. In some embodiments, the reaction product (i.e., the oxidation product) is an oxygenate of the starting compound, such as an alcohol, ketone, aldehyde, ester, or carboxylic acid.
When the acidic medium comprises a carboxylic acid (e.g., acetic acid, trifluoroacetic acid, or the like), the oxidation product must be separated from the acidic medium and from the electrophile reduction product. The electrophile reduction product can be recycled by an oxidative step back to the oxidizing electrophile, and the components of the acidic medium can be recycled back into the reaction milieu. However, the oxidation product needs to be economically recovered in sufficiently pure form in order to provide an efficient and/or cost-effective process for the overall functionalization (e.g., oxidation) reaction.